1. Field of the Invention
The invention is directed to an optoelectronic imaging system for industrial applications having at least two optical lenses which are spaced apart from one another and an electronic image sensor, wherein the imaging sharpness on the image sensor and the linear magnification or imaging scale (zoom) are mechanically adjustable by a control unit.
2. Description of the Prior Art
Optoelectronic imaging systems are contained in every video camera having focus and zoom capabilities. The objective lens of such a camera is generally outfitted with a device for manual and automatic focussing (autofocus) and for motor-controlled change of the imaging scale. The optical lenses of the objective are formed of solid glass bodies. To enable focussing and zooming, the axial relative distances between the individual lenses or lens groups in the objective are changed along the optical axis by means of an electric drive. Expenditure for this purpose is considerable not only with respect to the precision-mechanical drive technology, but also with respect to the space requirement for the objective (constructional volume). Infrared sensors are usually used to supply the measured values for the zoom function.
Particularly for industrial applications, the concern lies in providing devices that are as small and inexpensive to manufacture as possible. These devices can be used, for example, in the fields of security and process control, to protect plants or to observe processes. Optoelectronic imaging systems are also of great use in transportation, traffic control and manufacturing and assembly of products. For many applications, the requirements for exactness of image reproduction and resolving capability are lower than in commercial photographic and video devices, for example.
It has been known for many years that there exist possibilities for producing optical lenses whose focal length can be changed by flexible deformation of the lens body, that is, by changing the surface curvature. For example, U.S. Pat. No. 3,161,718 discloses flexible lens comprising two directly adjacent hollow bodies which are separated only by an optically transparent disk. The external surfaces of the two hollow bodies are formed by an optically transparent flexible membrane. The inner hollow body of the two hollow bodies is filled with a liquid which is likewise transparent to light. By increasing or reducing the filling volume of liquid, the curvatures of the surfaces and accordingly the focal lengths of the lens can be varied allowing for the atmospheric pressure acting externally on the membrane surface. For the purpose of further influencing the curvature shape, this reference describes an arrangement by means of which the pretensioning of the membrane walls can be changed independent from the pressure of the liquid filling.
A device designed as spectacles is known from GB 1 209 234, the glasses of these spectacles being constructed as flexible lenses which have a variable focal length, again through changing their liquid filling. Further, the publication "Silicone rubber applied within the eye: a preliminary study", Applied Optics, May 1, 1979, Vol. 18, No. 9, pages 1305 to 1310, describes the possibility of the medical use of flexible lenses which are intended for implantation in the human eye as a substitute for natural lenses. In this case, the lens is not provided with a liquid filling, but rather is formed of a solid elastic plastic material. Further, reference is had to the article "Variable-focus liquid-filled optical lens", Applied Optics, Aug. 1, 1993, Vol. 32, No. 22, pages 4181 to 4186, wherein the test results with respect to the imaging characteristics of liquid-filled flexible lenses are shown.
FR 26 34 287 describes an objective which has two or more liquid-filled lenses whose surface curvature can be changed manually by a dispensing or metering piston for the liquid which is adjustable by means of a screw.
U.S. Pat. No. 4,407,567 discloses an optoelectronic imaging system having three flexible lenses arranged at a fixed distance along the optical axis. The surface curvature of the flexible lenses can be changed in a controlled and reversible manner by means of piezo-actuators. A data processing device which applies a corresponding electric voltage to the piezo-actuators for adjusting defined surface curvatures is provided for this purpose. The data processing device is provided with two input devices through which actuating values for adjusting the imaging scale (zoom) and for focussing can be entered by the user of the imaging system. The flexible lenses and the actuating means form a device unit together with the data processing device. As can be seen from the further details given in the embodiment examples which work as alternatives to the flexible lenses with flexible concave mirrors, this optoelectronic imaging system can assume considerable dimensions since a distance between the outermost concave mirrors of 0.4 m to 3 m is indicated.
U.S. Pat. No. 4,802,746 discloses another optoelectronic imaging system in which the imaging is carried out on an electronic image sensor. The optics of this imaging system comprise two conventional lenses and a flexible lens whose surface curvature can again be changed in a controlled and reversible manner by piezo-actuators. The electric voltage for adjusting the piezo-actuators is regulated by a microprocessor. This microprocessor selects its actuating information for the voltage supply on the basis of the findings of a unit for determining image sharpness which detects the image sharpness by evaluating individual image sections during brief changes in the surface curvature of the flexible lens. In this system, which provides only one individual flexible lens, a zoom function would not be possible without axial displacement of the conventional lenses.